Could you use gravitational waves to communicate from inside the event horizon of a black hole to someone who was outside?
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1$\begingroup$ The event horizon is defined as the point where no information can escape - in other words, nothing within an event horizon can affect the rest of the Universe. So no communication is possible. Not to mention, how could we communicate with anyone using gravitational waves? They're just how gravity propogates. $\endgroup$– Sir CumferenceSep 8, 2016 at 14:07
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$\begingroup$ Is it no information? Or no light? Seems like significant energy is escaping a black hole when two are merging. Why couldn't that be a mechanism for sending information? $\endgroup$– joseph.hainlineSep 8, 2016 at 14:24
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4$\begingroup$ Possible answer in this physics question though you have to read between the lines a bit. $\endgroup$– AndySep 8, 2016 at 14:35
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$\begingroup$ @SirCumference how could we communicate with anyone using gravitational waves? They're just how gravity propogates. How could we communication with anyone using electromagnetic waves? They're just how electric and magnetic fields propagates. $\endgroup$– zephyrSep 8, 2016 at 20:46
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3$\begingroup$ @joseph.hainline It is no "information". Light just happens to be a form of information. And when two black holes merge, that energy in the form of gravitational waves isn't escaping from inside the black holes themselves. It is escaping from their orbital energy. $\endgroup$– zephyrSep 8, 2016 at 20:47
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1 Answer
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A black hole is defined as a region of spacetime where the gravitational effects are so strong that the escape velocity within the event horizon is greater than the speed of light. According to relativity, nothing can propagate faster than the speed of light, so at most gravitational waves would propagate at this speed. According to particle physics, mediation of gravity would be controlled by massless particles called gravitons, which, by virtue of being massless, would travel at the same speed as photons, i.e. the speed of light. Suffice to say gravitational waves (analogous to electromagnetic waves) would not be able to escape from inside the event horizon.
The question of information, in regards to black holes is open however. The No-Hair Theorem would seem to imply the destruction of information as the black hole consumes matter. However, the holographic principle, if correct would imply that information regarding the matter that formed the black hole would be encoded on the 2-dimensional boundary of the event horizon. This information could very well be exhausted by Hawking radiation or gravitational waves. Since this information was outside of the event horizon this would be fully consistent with the formulation of black holes.
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$\begingroup$ What would it mean for gravitons to not be able to escape the black hole? If that were true then what would cause gravity to be felt by things outside the black hole, if the mediating particle couldn't mediate because it couldn't escape the black hole? $\endgroup$ Oct 25, 2016 at 2:08